Remove and replace single player file

So I was thinking on do some low level gambling for an item. But that costs millions and I get broke... Can I take a copy of my save file, gamble and if I don't get it put my old save back?
I'm ok using a few mills getting the item, but hoped I didn't gave to re farm several times

Yes you can.

1) Go to your save folder (typically C:\Users\YourUserNameHere\Saved Games\Diablo II Resurrected)
2) Look for the .d2s file with the name of the character you'd like to use
3) Copy said file some place else
4) Do your gambling
5) Copy the backup version back over the one in the save folder to restore the prior state of the character

If you do get something useful along the way but still want to keep going, just make sure to drop it into your shared (not personal!) stash. That's a different save file so you keep it even when you restore the character to a prior state.

If you want to use your shared stash as well for extra gold "per restore", just do the same with SharedStashSoftCoreV2.d2i as you did with the charactername.d2s. Just keep in mind that that'll also then remove stuff you gained from gambling dropped into the shared stash post-copy (so grab it on a different character before restoring).

If you're planning to resort to such a cheat, why don't you just use the Hero Editor? Easier to use than manually copying save.

And, by the way, this is a common method to "resurrect" HC characters...

Ew